Serrodyne frequency shifters



e 3,058,049 EQ@ V Patented Oct 9, 1962 3,058,049 SERRDYNE FREQUENCY SHIFTERS Francis J. OHara, Belmont, and Howard Scharfman, Lexington, Mass., assignors to Raytheon Company, a corporation of Delaware Filed Mar. 30, 1959, Ser. No. 802,920 13 Claims. (Cl. 321-69) This invention relates to frequency shifting devices, and more particularly, to adjustable frequency shifting devices capable of producing discrete shifts in the frequency of energy traveling from a microwave source in an electromagnetic wave transmission system.

'In the past, klystrons and traveling wave tubes have been modulated by a voltage sawtooth to provide a frequency shift of the energy fed to such devices from an external source. Moreover, in addition to the modulating signal producing frequency shift, these devices generally require accurately regulated complex power supplies capable of supplying relatively high voltages. lIt is desirable, therefore, in numerous applications, to provide a single sideband or frequency translating device which requires relatively low input power and, at the same time, provides a discrete frequency shift of the carrier signal and any sidebands associated therewith.

In accordance with the invention, frequency translation of a microwave signal is accomplished by feeding linearlypolarized microwave energy through a two-mode transducer or thruplexer, having a pair of polarization-selective side arms or connections in either a round or square waveguide, and thence into a conventional circular waveguide polarizer. The resultant circularly-polarized electrical energy from said polarizer is fed into a ferrite phase Shifter comprising a longitudinally-magnetized ferrite rod axially positioned in the waveguide and is thereby phase shifted proportional to the applied longitudinal iield strength in the ferrite material. "'Ihe energy emerging from the ferrite phase shift section is directed to a shorting plate or ibar positioned at the end of the waveguide adjacent to the ferrite section, the sense of circular polarization of reilected electromagnetic wave energy being reversed by the shorting plate prior to reentering the ferrite phase shift section. The retiected wave moving back through the ferrite encounters a reversed longitudinal field so directed as to produce' a second value of phase shift in the same electrical direction as the first, the phase shift being doubled on the second trip through the ferrite phase shifter. In order to achieve frequency translation, a sawtooth modulating field current is applied to field coils surrounding the ferrite material. The sawtooth current wave applied to the ferrite lmaterial results in a longitudinal sawtooth magnetic iield which, in turn, causes a sawtooth phase shift in the microwave signal of an amplitude such that the signal under-goes a total phase shift of 360 degrees for each sawtooth modulation period. This is achieved by adjusting, in a conventional manner, the amplitude of the sawtooth current wave to a value which preferably produces ,360 degrees of phase shift. At this optimum value, the spectrum of the resulting phase modulated signal contains low level sidebands and a single high amplitude sideband which approaches the value of the original incident carrier. Thus the energy in the original carrier is displaced in frequency by a value equal to the frequency of the modulating signal. The circular polarizer then converts the circularly-polarized reflected and translated wave energy back to a linearly-polarized wave in a plane orthogonal or cross-polarized to the input eletromagnetic field, and in this manner, the side arm or second connection of the two-mode transducer can be used as the output of the signal translating device. ilt shouldbe noted that this serrodyne-type of modulating device is non-reciprocal in the sense that a positive displacement of frequency is realized for one Vdirection of propagation, and a negative displacement is obtained for the other direction. Moreover, the resultant quantity of frequency translation, so produced, is capable of being controlled by varying the frequency of the current sawtooth applied to the ferrite phase shifter.

In its broader aspects, the invention discloses a frequency shifting device comprising a current sawtooth modulated Wave fed to ferrite phase shifting devices capable of producing linear phase shift over a range of 360 degrees or greater. For example, devices of this type include a pair of ferrite rotators in which the iield producing coil surrounding each ferrite is connected in seriesI opposition to provide cancellation of rotation in the two devices while providing a phase shift depending upon the current in the iield producing coils.

The invention further discloses the use of a sawtooth modulating waveform applied to a single field coil surrounding a ferrite rod axially positioned in a rectangular waveguide.

The invention also discloses a modification of the above-described, cross-polarized phase shifter in which the input and output linearly-polarized magnetic fields are in the same plane, permitting the device to have a single port. This can be achieved by replacing the twomode transducer in the system with a fixed 45 degree ferrite Faraday rotator, or isolator, preceding the circular polarizer in the system. Thus, the signal reflected from this port is translated in frequency.

Other objects and advantages wil-l be more readily perceived upon analysis of the drawing, in which:

IFIG. 1 is an isometric View, partly in section, of the first embodiment of a frequency shifter according to the invention;

FIG. 2 is a sectional isometric view of a ferrite rod positioned in a rectangular waveguide in accordance with a further embodiment of the invention;

FIG. 2a is a sectional View of a ferrite rod positioned in a circular waveguide; and

FIG. 3 is a sectional View of a furthe-r embodiment of the invention.

Referring now to FIG. l, a frequency shifting system is shown which includes a two-mode transducer 12, a circular polarizer 14, a ferrite phase shifter 16, and a shorting bar 1S, positioned at the end of the waveguide adjacent to the ferrite phase shifter. As indicated, the fer- -rite phase shifter 16 with the exception of the waveguide surrounding the ferrite is similar to the rotator section of the phase shifter disclosed in the co-pending application of Howard 'Scharfmam Serial No. 648,897, tiled March 27, 1957, and assigned to the assigneeof this invention. This waveguide will be described in detail. More particularly, FIG. l shows a rectangular input section of waveguide 20 having a flange 21 adapted to provide a connection with a microwave source of energy, such as a magnetron or klystron oscillator. The rectangular waveguide 20 accepts and supports only 'DE01 waves in which the electric vector, which determines the plane of polarization of the wave, is parallel to the short side of the rectangular waveguide.

The two-mode transducer 12 is shown integral with the input section of waveguide 20. The transducer includes a rectangular output arm 23, a flange 24 adapted to connect microwave energy to a load and the input section 20 so oriented relative to the circular waveguide section 25 as to provide a pair of conjugately-related terminals or branches in a manner that a wave launched in either one will not appear at the other. Therefore, microwave energy of the TEM mode introduced into waveguide 20 will flow through the two-mode transducer 12 into the circular polarizer 14.

It should be understood that any standard type circular polarzer may be used, provided the proper polarization and mode of energy is thereby obtained. Moreover, it is obvious to one skilled in the art that a number of other well-known circular polarization means may be employed in lieu of the circular polarizer 14 as long as the length of the polarizer, and the structure within the section of waveguide is adapted to convert linearly-polarized energy to circularly-polarized energy. In the present instance, circular polarizer 14 comprises a section of circular waveguide 25 having a card or vane of dielectric material 27 diametrically positioned at 45 degrees to the plane of the linearly-polarized input field within the circular waveguide for the distance necessary to convert the linearly-polarized field to a circularly-polarized field. The resultant circularly-polarized field is connected by means of output flange 28 to the ferrite phase shifter 16, which has the property of phase shifting the microwave energy transmitted therethrough in a direction which is dependent upon the direction of the applied axial magnetic field. More particularly, therefore, the phase shift through the ferrite phase shifter is described in detail in an article by Howard Scharfman entitled Three New Ferrite Phase Shifters, in the Proceedings of the I.R.E., vol. 44, pp. 1456-1459, October 1956.

The ferrite phase shifter 16, connected to the circular polarizer 14, includes a cylindrical ferrite element 32 positioned within the waveguide section which preferably consists of a low-loss dielectric material 33 such as wellknown Kel F or Rexolite which acts as a solid-supporting medium for the ferrite element 32. This dielectric material is coated with silver paint 34 to form a waveguide with sufficiently low conductivity so as not to constitute a shorted turn to the modulation energy in the field coil surrounding the ferrite. The dielectric material is center drilled to accept the ferrite rod 32. Many other methods for mounting the ferrite within the dielectric will suggest themselves to those skilled in the art.

The ferrite device 16 further includes a magnetic fieldproducing means, such as field coil 36, surrounding the circular waveguide 34 in the region of the ferrite element 32. The field coil is pie Wound for low distributed capacity and consists of 1,000 turns of wire connected to a conventional sawtooth wave current generator 37 which is variable in amplitude and frequency. In this embodiment, the sawtooth wave generator 37 is variable in frequency from approximately one cycle to fifty kilocycles and from approximately 100 milliamperes to 250 milliamperes. In the present instance, the sawtooth wave 9 is approximately 175 milliamperes peak-to-peak at a frequency kilocycles. This current is applied to :field coil 36 by way of leads 38 and 39. A detailed discussion of the invention has been published by the Institute of Radio Engineers Transactions on Microwave Theory and Techniques, January 1959, in vol. MT1`7, beginning on page 32.

As shown in-the diagrammatic representation of the path lengths followed by electrical energy traversing the phase shift system of FIG. l, a vertically-polarized wave shown at 41 is circularly-polarized by the circuit polarizer 14 in the manner represented at 42, is phase shifted a predetermined number of degrees represented by the vector at 43, and reflected from shorting bar 40. The reflected wave moving back through the ferrite experiences a second phase shift of substantially the same value as the first phase shift, as represnted at 44, and consequently, the phase shift is doubled on the second trip through the ferrite element.

As noted, the refiected wave moving through the ferrite phase shifter reenters the circular polarizer 14 and is converted from a circularly-polarized reflected wave back to a linearly-polarized wave in a plane cross-polarized to the input eld as represented by the arrow 45 of FIG. 1. In this manner, the side arm 23 of the thruplexer 12 may be used as the output of the frequency shifting system.

It should be understood that the ferrite phase shifter 16 can be constructed so as to have two planes of symmetry. It is possible, of course, to utilize a ferrite element of square outer configuration in conjunction with a circular waveguide or vice versa. It is also understood that the thruplexer 12 may consist of the waveguide of either square or circular cross-section as long as the dimensions of each side arm are chosen so that only the dominant mode in each can be propagated. It should be further understood that the length of the ferrite element 32 preferably exceeds that of its diameter so that the required magnetic field for a given angle of phase shift is held relatively low. It is also desirable to maintain the diameter of the ferrite rod less than l the diameter of the waveguide 33 for low-loss attenuation of the propagated wave. As noted, a square or circular waveguide structure including symmetrical ridged guides in which the circularly-polarized waves of opposite sense have the same phase constant at zero field strength can be used as the propagation medium.

Referring to FIG. 2, a generator of a sawtooth current 6) is shown feeding a magnetic field producing coil 61 surrounding a ferrite rod 62 positioned in a rectangular waveguide 63 by means of a low dielectric support material 64. This combination provides relatively great values of phase shift in a small physical package which, in turn, provides efficient frequency translation in the aforementioned manner. Referring to FIG. 2a, a generator of a sawtooth current 70 is connected to a circular waveguide 71 which is provided with a pair of field coils, 72 and 73, surrounding the guide in the region of a ferrite element 74. This latter element is supported in the guide by dielectric material 75. The coils are connected in series opposition and fed by a sawtooth wave. In this manner, the two opposing longitudinal magnetic fields produced by the oppositely-connected eld coils produce opposing magnetizations in the adjacent portions of the ferrite rod and provide that the total rotation of the plane of polarization through the two rotators is zero, independent of the current through the coils. However, the phase shift through the two rotators is twice that for each rotator, and a frequency shift is obtained from the current sawtooth modulating wave input to the coils, as noted, according to the repetition frequency of the sawtooth wave. The phase shifter is described in detail in the aforementioned article on ferrite phase shifters by Howard Scharfman.

Referring now to FIG. 3, a modification of the single ferrite device of FIG. l is shown wherein a non-reciprocal 45 degree ferrite Faraday rotator 48 of conventional design may be inserted between the circular polarizer 14 and thruplexer 12a, the side arm 23a of the thruplexer being terminated in a well-known matched termination 47, such as a section of waveguide filled with Polyiron, or any similar non-reflecting material adapted to absorb electrical energy. As shown, the 45 degree ferrite rotator 48 comprises a section of circular waveguide 49 filled with Teflon or similar dielectric material 29 and supporting a second ferrite element 50 axially positioned within the low-loss dielectric material. A separate magnetic fieldproducing means 51 surrounds the circular waveguide section 49 in the region of the ferrite element S0. This field-producing means may consist of a permanent magnet of sufficient strength to produce a non-reciprocal 45 degree rotation of the microwave energy passing through the ferrite material in the forward and reverse directions. This angular displacement of the linearly-polarized fields is represented by the arrows 52 and 53 in FIG. 3. In like manner, the input and output energy linearly-polarized in the same plane is represented by the arrows 41 and 55. Consequently, the device may be used as a reflection type or one-port frequency shifter.

While the shorting bar 18, as shown in FIGS. 1 and 3, is located a short distance from the ferrite element 32, it should be understood that at low frequency modulation of the ferrite, the field coil 36 may be located proximate or in contact with the shorting plate 18. This is possible because the eddy current eects produced in the shorting bar 18 are negligible. However, at high modulation frequencies, the shorting bar is preferably positioned, as shown, a short distance from the eld coil 36 and the ferrite element 32 to prevent eddy current losses from dissipating the driving field energy applied to the phase shifter. The waveguide 3S, as shown in phase shifter 16a, may consist of a metal tube rather than a silver coating. Also, a smaller field coil is preferably used to produce the specified phase shift when it is desired t0 produce a frequency translation with modulation waveforms having appreciable high frequency components.

For the foregoing reasons, it is to be understood that the above-described arrangements are illustrative of the application of the principles of the invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention. Moreover, the invention is not limited to the particular details of construction, materials and processes described, as many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

l. In combination, a section of waveguide adapted to receive and support electromagnetic energy, ferrite element means positioned in said waveguide section adapted to propagate said electromagnetic energy, means for translating said electromagnetic energy positioned adjacent to said ferrite element means for providing a .predetermined phase shift of electromagnetic energy propagated in said ferrite element, said latter means including a series of sawtooth current waves adapted to provide a phase shift of 360 degrees amplitude.

2. A device for producing a predetermined frequency shift in an electromagnetic wave comprising a section of waveguide adapted to receive an incident carrier wave of electromagnetic energy, a ferrite element positioned within said waveguide, means including a pair of field coils connected in series opposition surrounding said waveguide in the region of said ferrite element for directing opposing magnetic fields into said ferrite element, and means for applying a sawtooth current Wave form to said field coils of an amplitude to produce a 360-degree sawtooth phase shift thereby to provide a single sideband which approaches said incident carrier Wave in amplitude.

3. In combination, a section of waveguide adapted to receive and support electromagnetic energy, ferrite element means positioned in said waveguide adapted to propagate said electromagnetic energy, means adjacent to said ferrite element means to magnetize said ferrite element in a manner adapted to provide a predetermined phase shift of 360 degrees in the electromagnetic energy propagated in said ferrite element, said latter means including a series of sawtooth current waves of a predetermined amplitude to produce said phase shift by a saW- tooth variation in the magnetization of said ferrite element.

4. A frequency shifting system for producing a frequency shift in electromagnetic energy travelling through a waveguide transmission line comprising means for feeding linearly-polarized input wave energy into said waveguide transmission line, rotator means for rotating said linearly-polarized energy through yan angle of 45 degrees, means for converting said rotated wave energy to circularly-polarized wave energy, means for introducing a first phase shift in said circularly-polarized wave energy, means for reflecting said phase shifted wave energy through said phase shifting means in a second direction to produce an additional phase shift in said wave energy in the same electrical direction as said first produced phase shift, said means for introducing said first and second phase shift including a sawtooth current wave to' produce a frequency shift by a shift of substantially 360 degrees from said phase shifted wave energy, and means for feeding said frequency shifted Wave energy through said circular polarizer and rotator means, whereby said frequency-shifted wave energy is rotated to substantially the same electric polarization angle and is substantially the same amplitude as said linearly-polarized input wave energy.

5. A device for producing a predetermined frequency shift in an electromagnetic wave comprising a section of waveguide adapted to receive an incident carrier wave of electromagnetic energy, a ferrite element positioned within said waveguide, means including a pair of field coils surrounding said waveguide in the region of said ferrite element adapted to direct opposing magnetic fields into adjacent portions of said ferrite element, and means for applying a sawtooth current waveform to said field coils to provide a 360 degree phase shift to produce a single sideband signal in said ferrite element of an amplitude which approaches said incident carrier wave of electromagnetic energy.

6. A frequency shifting system for producing a frequency shifty in electromagnetic Vwave energy travelling through -a waveguide transmission line comprising means for feeding circularly-polarized electromagnetic wave energy into a phase-shifting means in one direction, said phase-shifting means including a waveguide section containing a ferrite element, means for applying a longitudinal magnetic field to said ferrite element to produce a 360 degree phase shift to provide only low level sidebands and a single high amplitude sideband, reflecting means directing said phase-shifted wave energy through said ferrite element in another direction, said means for applying said longitudinal magnetic field including a sawtooth current waveform to produce a single sideband signal in said ferrite element.

7. In combination, a section of waveguide adapted to receive and support electromagnetic energy, ferrite element means positioned along the longitudinal axis of said waveguide, means adjacent to said ferrite element means for providing polarization rotation of electromagnetic energy propagated in said ferrite element in equal and opposite angular directions, means for electrically changing the angle through which said rotation occurs, said latter means comprising a series of current sawtooth waves of an amplitude adapted to provide a predetermined phase shift of 360 degrees for each sawtooth wave in said section of waveguide.

8. A device for producing a predetermined frequency shift of a circularly-polarized electromagnetic wave comprising a wave-guide receptive of said electromagnetic wave, a ferrite element positioned along the longitudinal `axis of said waveguide, a field coil positioned in the region `of said ferrite element, a sawtooth current wave generator electrically connected to said field coil for providing a modulating magnetic field of `sufficient amplitude to produce a ,frequency shift by a phase shift of at least 360 degrees in said electromagnetic wave, and a shorting element terminating said waveguide adjacent to said ferrite element, thereby reflecting said electrom-agnetic wave back through said ferrite element.

9. A device for producing a predetermined frequency shift in a circularly-polarized electromagnetic wave entering said device comprising a waveguide receptive of said electromagnetic wave, a ferrite element positioned along the longitudinal axis of said waveguide, a iield coil positioned in the region of said ferrite element, a sawtooth current wave generator connected to said field coil for providing a sawtooth modulating magnetic field parallel to the direction of propagation of said electromagnetic wave, the strength of said modulating magnetic eld being below that required for ferromagnetic resonance and of sufficient magnitude to provide a phase shift of substantially 360 degrees in said electromagnetic wave, and shorting means terminating said waveguide adjacent to said ferrite element, thereby reflecting said modulating energy back through said ferrite element.

10. A device for producing a predetermined frequency shift of an electromagnetic wave having a predetermined first frequency entering said device comprising a rectangular waveguide receptive of said electromagnetic wave, a ferrite element positioned along the longitudinal axis of said waveguide, a field coil positioned in the region of said ferrite element, a sawtooth current wave generator connected to said field coil for providing a sawtooth modulating magnetic field of sufficient amplitude to produce a phase shift of substantially 360 degrees in said electromagnetic wave entering said ferrite element, whereby the amplitude of said electromagnetic wave is substantially unchanged and the frequency thereof is displaced from said rst frequency of said electromagnetic wave by a value equal to the frequency of the modulating magnetic field.

11. A device for producing a frequency shift of a polarized electromagnetic wave entering said device comprising a circular waveguide receptive of said electromagnetic wave, means for converting said electromagnetic wave from linear to circular polarization in said waveguide, ferrite element means positioned along the longitudinal axis of said waveguide receptive of said circularlypolarized electromagnetic wave, means positioned in the region of said ferrite element for providing a magnetic field parallel to the direction of propagation of said electromagnetic wave, means for producing a sawtooth current wave of an amplitude to produce a phase shift of 360 degrees in said electromagnetic wave entering said ferrite element.

12. A device for producing a frequency shift of a circularly-polarized electromagnetic wave comprising a circular waveguide receptive of said electromagnetic wave of a given amplitude, a ferrite element positioned along the axis of said waveguide, a field coil positioned in the region of said ferrite element, means for supplying a sawtooth current wave to said field coil of an amplitude such that said electromagnetic wave is provided with a total 3 phase shift of substantially 360 degrees for each sawtooth modulation period, and waveguide shorting means reflecting said wave energy of substantially the given amplitude back through said ferrite element in the reverse direction.

13. A frequency shifting device for electromagnetic wave energy comprising a section of waveguide adapted to receive and support electromagnetic energy in a plurality of linear polarizations, a pair of polarization-selective connections at one location along said waveguide each adapted to be coupled to an orthogonal polarization of linearly-polarized wave energy at said location, one of said selective connections adapted to be connected to a load circuit, the other of said selective connections being connected to a section of waveguide adapted to convert linearly-polarized energy lfed from said other connection to circularly-polarized energy, a ferromagnetic element adapted to produce a phase shift of said circularly-polarized energy positioned in said sectional waveguide, a sawtooth current generator adapted to produce a sawtooth magnetic field in the region of said ferrite element of an amplitude adapted to provide phase shift of substantially 360 degrees in said electromagnetic energy in said ferromagnetic element, and a reflecting member located at the end of said guide adjacent to said phase shift element to present a substantially short circuit to wave energy incident upon said reecting member, whereby said phase shifted energy is reected back through said ferromagnetic element to the other of said selective connections.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Three New Ferrite Phase Shifters, by Scharfman, in Proceedings I.R.E. (October 1956), pages l457l459. 

